In recent years, a digital image system has been amazingly developed, and construction of digital image processing technology has been progressed. For example, in the field of a copying machine and a multifunction printer each using an electrophotographic method or an ink-jet method, it is further requested to store documents as electronic data and to manage thus stored document files. Moreover, in the field, documents are compressed and transmitted via e-mail.
A scanned image that is scanned by a scanner generally has a large file size. Accordingly, an image compression technique is indispensable in storage and transmission of the scanned image. One image compression technique for achieving a high compression efficiency is based on layer separation. According to the image compression technique based on the layer separation, an image is separated into a foreground layer and a background layer, each of which is then compressed by using a compression technique appropriate for each of the foreground layer and the background layer. Ultimately, a highly compressed image is generated. The foreground layer contains text and line drawing, and is generally compressed by a lossless compression technique such as JBIG (Joint Bilevel Image Group), MMR (Modified Modified Read Code), or LZW (Lempel Ziv Welch).
On the other hand, the background layer contains image contents other than the text and line drawing and is generally compressed by using a lossy compression technique such as JPEG (Joint Photographic Experts Group). Because a compression efficiency of the lossy compression technique is easily controlled, file size or image quality can be selectively given priority depending on application. Meanwhile, a compression efficiency of the lossless compression technique is hard to control.
Patent Literature 1 discloses a technique as a method for improving a data compression efficiency in a case where such compression techniques are used. According to this technique, an inputted image is subjected to segmentation so that the inputted image is separated into line drawing image data and halftone image data. The line drawing image is stored in a memory directly or after implementation of reversible conversion, whereas the halftone image data is stored in the memory after implementation of lossy compression using frequency conversion. In this way, the data is compressed.